Monge’s reference system consisted of a vertical plane (V in Figure 2A ) and a horizontal plane (H) that intersected in a ground line. As in Figure 2A , Monge numbered the four quadrants formed by V and H I, II, III, and IV. Figure 2A also shows two arrows, D1 perpendicular to H and D2 perpendicular to V. Each arrow represents the direction of projection from points on any object under study to one of the reference planes. Such an object is the L-shaped block located in the first quadrant. Monge introduced the concepts of the reference system, the formation of views by projectors perpendicular to the reference planes, the revolving of the H plane into coincidence with the V plane about the ground line as indicated by the curved arrows, and the retention of the images on the planes after the object had been removed and the H plane revolved. Figure 2B illustrates the final result: the projection on V is regarded as the front view, and the projection on H as the top view.

If the object were placed in the third quadrant (see Figures 2C and 2D ), the projections would be exactly the same, but their relative locations on the paper would be reversed. If the object were located in the second quadrant, the two projections would have the same shape and size as in Figures 2B and 2D . Depending on the location of the object in the second quadrant, however, now either projection might be located above the other or one projection might overlap the other. The same is the case if the object were located in the fourth quadrant. This uncertainty is the reason that commercial use is limited to first- or third-quadrant projection. First-quadrant projection is often referred to as first-angle projection, and third-quadrant projection as third-angle projection.

Regardless of the quadrant (or angle) used, the views or projections are formed by the intersection of the projectors and the reference planes. Established conventions determine which points of the object are projected. If projectors were extended from every point on the object to the reference planes, the views would be silhouettes and would fail in their purpose of defining the object. The accepted rule is to project (1) all points on the edges between plane surfaces that bound the object and (2) all points at which the projectors forming the view are tangent to curved surfaces of the object. Figure 3 illustrates these two sets of points. AB is the line of intersection of the cylindrical surface and plane surface ABCD. CB is the line of intersection of two plane surfaces. EF is not the line of intersection of two surfaces of the object, but projectors forming the top view are tangent to the cylindrical surface along the straight path from E to F, and thus EHFH properly appears in the top view. (The superscript H is used here to denote the projection on the H plane, and, similarly, V is used to denote the projection on the V plane.) CD is the line of intersection of the cylindrical surface and plane ABCD, but CHGH results from the tangency of projectors along the cylindrical surface. Every line projected in the identical front view of this object is a line of intersection of surfaces. AD, BC, and the plane ABCD all project as the same straight line in the front view because the plane ABCD is parallel to the projectors for that view.

Ambiguity must be avoided in the views, dimensions, and notes of a set of drawings. Figure 4A shows pictorial representations of three different objects for which the identical front and top views in Figure 4B are correct. The ambiguity in the shape description provided by front and top views alone can be eliminated by adding a third, or side, view obtained by projecting the object onto a vertical plane perpendicular to V. In Figure 4B each set of three views describes only one of the objects without ambiguity.

In commercial or industrial practice, sets of drawings ordinarily provide at least three views of any part that is not a stamping, a gasket, a flat wrench, or other essentially two-dimensional form. Depending on the shape of the part, there may be a left-side view, a right-side view, or both. There may be reason for a back view, a bottom view, or both. Additional views are discussed below.

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